The variable beak of the finch: How hybridization increases variation in beak morphology

Darwin’s Finches show more variation in beak morphology due to introgressive hybridization.

In 2015, I attended the workshop “Evolutionary Biology in Guarda”. The idea of this workshop is to develop a research project on an evolutionary topic under the guidance of several well-known evolutionary biologists. That year, the teaching staff consisted of Peter and Rosemary Grant, Richard Lenski, Dieter Ebert and Sebastian Bonhoeffer. It was an inspiring time: thinking about evolution in the idyllic Swiss mountains.

I remember one session where Peter and Rosemary Grant visited our working group. After listening to our ideas, Peter stared out the window and said: “Look at that red flower. Why is it red? You can think of several explanations. For example, it might be red to attract insects for pollination. That would be your hypothesis. Now you could devise an experiment to test this hypothesis. From the experiment, you can then test several predictions.” With this short musing, Peter wanted us to think about the difference between hypotheses and predictions. And it is nice to see that he has followed his own advice and published a study in the journal PNAS (together with Rosemary Grant, obviously) in which a clear hypothesis is formulated and certain predictions are tested.

Geospiza_conirostris

The Large Cactus Finch © Harvey Barrison | Wikimedia Commons

 

A Hypothesis on Hybridization

Let’s look at the hypothesis first, outlined in the following scenario: “When populations begin to diverge, an exchange of genes may be frequent but will have little effect on the variation of each. As morphological divergence proceeds further to a point at which the populations become biological species—they seldom interbreed but suffer little or no loss of fitness when they do—phenotypic and genetic effects of gene mixing are expected to be greater, and at some point reach a maximum. Thereafter, population variation declines, caused by strengthening of premating isolating mechanisms and hence increased rarity of interbreeding, and/or by the accumulation of incompatible alleles through mutations that reduce or prevent exchange.”

From this scenario, we can formulate a general prediction: the population variation in traits affected by hybridization should increase with time, reach a peak and then decline. Peter and Rosemary tested this prediction using data from Darwin’s Finches on the Galapagos Islands. They studied data on beak morphology from several populations across the archipelago. When they plotted the average coefficient of variation in beak length versus the age of the species, the predicted pattern arose.

beak_age

The variation in beak length shows the expected pattern in accordance with the scenario of hybridization dynamics. From: Grant & Grant (2019) PNAS

 

Common Cactus-finch

Apart from the general prediction, we can also focus on more specific cases. Does the variation in beak morphology also increase on a species level? To answer this question, the researchers turned to the small island of Daphne Major where they have been monitoring several hybridizing species.

Here, the Common Cactus-finch (Geospiza scandens) interbreeds with the Medium Ground-finch (Geospiza fortis). Because the Common Cactus-finch is the largest species, we expect its beak to become smaller over time (i.e. more like the beak of a Medium Ground-finch). In addition, the variation in beak morphology is expected to increase over time due to introgressive hybridization. And that is exactly what we see: beak length clearly decreases (first figure below) and variation in beak length increases (the black line in the second figure below) over time.

Interestingly, the effect on beak depth is a bit different. This implies that the effect of hybridization on beak length and beak depth (the red line in the second figure below) is uncoupled. These traits are correlated, but seem to be evolving independent to some degree.

scandens

The effects of hybridization on beak morphology. The beak length decreases (top) and the variation in beak length increases (bottom, black line) over time. The sudden shift around the year 2000 is due to an El Niño-effect when there was no breeding on the island. From: Grant & Grant (2019) PNAS

 

Empirical Evidence

This study nicely illustrates the power of empirical data. It makes intuitive sense that introgression can result in increased variability. Indeed, several modelling studies have illustrated these predictions (see for example this blog post). However, actually testing the predictions with field data is another story. Peter and Rosemary Grant show how the yearly meticulous collection of population-level data (from 1973 to 2012!) can help evolutionary biologists answer outstanding questions.

In addition, they illustrate the evolutionary importance of hybridization in creating variation. The final sentence of the paper nicely captures this conclusion: “Hybrids are more live paths to the future than dead ends.”

Guarda_2015

The attendants and teaching staff at the workshop in Guarda. Can you find Peter and Rosemary Grant? © Jente Ottenburghs

 

References

Grant, P. R., & Grant, B. R. (2019). Hybridization increases population variation during adaptive radiation. Proceedings of the National Academy of Sciences116(46), 23216-23224.

 

This paper has been added to the Thraupidae page.

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